Typically, such a cover assembly comprises an annular cover having a flange extending radially from its outer periphery for its securement to its associated reaction plate, an annular diaphragm spring having a Belleville washer outer portion and a central portion divided into radial fingers acting as release levers, assembly means for rockably mounting the diaphragm spring on the cover, and a generally annular pressure plate fixed for rotation with and axially displacement with respect to the cover.
In operation the diaphragm spring has a first fulcrum or pivot referred to hereinbelow as the primary fulcrum or pivot, on the cover, or a part fixed thereto which urges the pressure plate toward the reaction plate so that the driven disc, clamped between the reaction plate and the pressure plate is fixed for rotation therewith; the clutch is then in its engaged position.
For disengagement, an axially displaceable part known as a clutch release bearing or throwout bearing must apply to the ends of the radial fingers of the diaphragm spring relatively adjacent to the axis of the cover assembly axis a force, known as the release or disengagement force, adapted to relieve the force the diaphragm spring applies to the pressure plate, thereby freeing the driven disc.
Owing to the structure of the Belleville washer peripheral portion which essentially comprises the resilient means of the diaphragm spring, the diaphragm spring has, as is known, a release load characteristic, measured in line with the ends of the radial fingers which for a given length of travel, hereinbelow referred to as the release travel, of the ends of the radial fingers, in response to the disengagement force applied by the release or throwout bearing, and from a given initial position of the radial fingers, goes through a peak or maximum value before reaching its ultimate or final value at the end of release travel, for the final position of the ends of the radial fingers.
The curve representing the release load as a function of release travel therefore has a characteristic saddleback or hump.
In practice, in the course of the service life of a diaphragm clutch, the peak value and the final release load of the diaphragm spring vary. Indeed, they depend on the overall configuration of the diaphragm spring in the engagement position of the clutch, and therefore depend particularly on the degree of wear of the friction facings of the driven disc, the pressure plate in the engaged position, which engaged position depends on the configuration of the diaphragm spring coming closer to the reaction plate as the friction facings become worn.
It is thus possible to say for the cover assembly taken on its own that the peak value and the final value of the release load on the diaphragm spring also depend on the axial distance, hereinbelow referred to as the reference distance, which at the initial position of the ends of the radial fingers of the diaphragm spring at the start of release travel moves the pressure plate away from the radial edge of the cover remote from the diaphragm spring. This reference distance, in practice, may vary between a maximum reference distance for a new driven disc and a minimum reference distance for a worn driven disc, or even in the absence of the driven disc when the cover assembly is in its stored condition.
One of the major problems posed with diaphragm clutch cover assemblies of the foregoing type relates to the assembly means rockably mounting the diaphragm spring on the cover. Indeed, while, as mentioned above, the diaphragm spring bears against a first fulcrum or pivot referred to as the primary fulcrum, on the cover or a part bearing against it, the assembly means must define a second pivot or fulcrum, referred to as the secondary fulcrum, as the same radius as the primary fulcrum while permitting rocking movement of the Belleville washer peripheral portion of the diaphragm spring when it shifts from its engagement position to its disengagement position of the clutch.
In practice, the assembly means rockably mounting the diaphragm spring on the cover exert a predetermined resilient clamping load between the primary and secondary fulcra.
According to a first option, which is the case with U.S. Pat. No. 3,323,624 (Maurice) issued to the assignees of the present application on June 6, 1967, this clamping load is, by construction, always less than the release load for the diaphragm spring, thereby permitting accentuated progressiveness from the engagement position to the disengagement position of the clutch.
This arrangement, which has given and continues to give satisfaction, at least for certain application, and which is characterized by the separation of the diaphragm spring from the first and second fulcra during disengagement, develops axial play between the diaphragm spring and its fulcra, which is not only detrimental to pressure plate lift from the reaction plate at the end of release travel, and therefore detrimental to correct unclamping of the driven disc, but is also a source of noise and wear.
According to a second option, as disclosed in French Pat. No. 1,599,075 granted on July 15, 1970, the resilient clamping force applied to the diaphragm spring by the assembly means rockably mounting it on the cover, is, by construction, always greater than the clutch disengagement or release load so that such play may not develop.
This arrangement which is therefore characterized by constant contact between the diaphragm spring and its primary fulcrum and thereby contributes to satisfactory lift off of the pressure plate at the end of release travel, has the drawback of interfering with freedom of movement of the diaphragm spring which is necessary for proper smooth rocking from its engagement position to its disengagement position.
Furthermore, by reason of considerable friction due to the resilient clamping involved in this arrangement, an important "hysteresis" effect is inevitably introduced into the assembly, particularly at its actuating end. Finally, the overloading of the diaphragm spring by the fulcra which results, causes increased wear thereof.